Tennis racket frame made of metal oxide fibers and ceramic particles

ABSTRACT

Improved tennis racket in which an inverted chevron shaped yoke is provided having downward sloping side portions which match the curvature of the arcuate top portion of the racket head to provide vertical strings extending between the yoke and arcuate top portion which are of the same length. The equal length of vertical strings provide uniform elastic response to ball impact. The particular design of the chevron provides added structural strength to the neck of the racket. An improved racket made from a polycrystalline metal oxide fiber and resin matrix is also disclosed. In addition, an improved string protection system involving elastomer inserts which are molded into the head portion of composite material rackets is described.

BACKGROUND OF THE INVENTION

The present invention relates generally to tennis rackets--their design,construction and manufacture. More particularly, the present inventionrelates to an improved tennis racket having a head portion shaped foroptimum performance and an improved string mounting system. The presentinvention also relates generally to materials and methods for makingcomposite material tennis rackets. More particularly, the presentinvention relates to a tennis racket frame made from ceramic fibercomposite materials which exhibit increased durability and wearresistance while still providing a racket frame which is lightweight,strong and flexible.

Tennis rackets basically include a head portion, a handle and a neckportion which connects the handle to the head. In general, one or morestructural supports or yokes are provided which extend across the neckportion in order to strengthen the racket and provide a location forstrings to be mounted at the bottom of the racket head. Numerousdifferent head designs have been proposed in the past to provide aracket with optimum performance characteristics. It usually is desirableto provide a racket which has as large a "sweet spot" as possible. The"sweet spot" of a racket is that area of the racket head which whencontacted by the ball during the tennis swing provides optimumreproducible and controllable hitting by the player. It would bedesirable to provide a tennis racket head which increases the size ofthe sweet spot present in the racket head. It would also be desirable toprovide a yoke structure extending across the racket neck which increasethe strength and structural integrity of the racket neck.

Another important consideration in designing a tennis racket frame isthe particular material utilized in fabricating the racket. Wood andmetal racket frames have been in common use for many years. Morerecently, racket frames made from composite materials such as resinimpregnated graphites, glass and boron fibers have become available.Composite material rackets made from graphite fibers have beenparticularly popular due to their high strength and light weight. Oneproblem with graphite rackets is that they tend to be easily abradedwhen struck against the tennis court or other hard surfaces. It would bedesirable to provide a composite material racket having the desirablelight weight and structural strength of graphite while being moredurable and resistant to abrasion. Graphite rackets also tend to be veryrigid and inflexible. In general, a certain amount of racket flexing isdesired during and after ball contact. It would be desirable to providea composite material racket which is more flexible than graphite fiberrackets.

As is well known, composite material rackets are generally made byforming uncured, resin impregnated fibers into a tennis frame precursorand then curing the composite material to form the finished tennisracket frame. In the head portion of the tennis racket, string holesmust be provided for mounting the tennis strings. In general, the stringholes are molded as part of the frame or drilled into the tennis frameafter curing. Protective grommets or a protective grommet strip is thepositioned around the head portion to protect the strings as they passthrough the string holes from abrasion, cutting and breakage. It wouldbe desirable to provide a tennis racket structure in which some othermeans for protecting the tennis strings is provided other than theindividual grommets or grommet strips presently in use.

It is an object of the present invention to disclose and provide atennis racket which is made from a composite material which providesequal or better performance characteristics than graphite compositerackets while providing increased durability and abrasion resistance.

An additional object of the present invention is to disclose and providea tennis racket having a head portion with an improved string mountingconfiguration which is easily manufactured and provides optimumprotection of the tennis strings at the point of contact with the tennisframe.

SUMMARY OF THE INVENTION

The present invention is an improvement upon a tennis racket framehaving a head, a handle and a neck extending between the head andhandle. The head of the tennis racket frame has an arcuate top portionand two spaced arcuate side portions extending between the neck and thearcuate top portion of the head. The neck includes two arcuate memberswhich extend between the arcuate side portions of the head and thehandle. The neck further includes a yoke extending between the arcuatemembers of the neck. The arcuate top portion, arcuate side portions,neck and yoke of the tennis racket frame define a ball contact zoneacross which a plurality of spaced vertical strings and spacedhorizontal strings are strung.

A feature of the present invention is the provision of a tennis racketmade from polycrystalline metal oxide fibers in a resin matrix. It hasbeen found that polycrystalline metal oxide fibers, such as the socalledceramic fibers composed of aluminum oxide, boron oxide and silicondioxide, when combined with a resin matrix selected from the groupconsisting of silicon, epoxy, phenolic and polyamid resins provide aparticularly desirable tennis racket frame material which is lightweightand abrasion resistant.

An additional feature of the present invention involves incorporatingceramic particles such as boron carbide, silicon carbide or titaniumcarbide into the polycrystalline metal oxide fiber/resin matrix toprovide a tennis racket which is especially resistant to abrasion whilestill being lightweight, flexible, and strong. Further, rackets madefrom the polycrystalline metal oxide fibers in a resin matrix inaccordance with the present invention provide a racket having optimum"feel" and performance characteristics.

As another feature of the present invention, a tennis racket frame madefrom composite materials is provided in which the string holes are madeby molding rod shaped elastomeric inserts into the head portion duringcuring of the composite material racket frame to provide a curedcomposite material racket having a plurality of elastomeric or plasticinserts molded therein. The string holes are made by either molding ordrilling holes through the elastomeric inserts prior to or after moldinginto the rachet frame to provide string mounting holes having a moldedin plastic protective grommet between the string and the compositematerial. This provides an especially convenient and effective means forprotecting the racket strings from abrasion against the much hardercomposite material and the resultant premature failure of the strings.

The above described and many other features and attendant advantages ofthe present invention will become apparent as the invention becomesbetter understood by reference to the following detailed descriptionwhen considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a preferred exemplary racket n accordance withthe present invention.

FIG. 2 is a front view of the preferred exemplary racket shown in FIG.1.

FIG. 3 is a side view of the preferred exemplary racket shown in FIG. 1.

FIG. 4 is a bottom view of the preferred examplary racket shown in FIG.1.

FIG. 5 is a sectional view of FIG. 2 taken in the II--II plane.

FIG. 6 is a perspective view with a partial cross section showing anexemplary precursor composite frame having the elastomeric or plasticstring inserts placed therein prior to molding.

FIG. 7 is a sectional view of the partial frame precursor shown in FIG.6 in position within the curing mold prior to curing.

FIG. 8 is the same view as FIG. 7 showing the cured composite frameinside the mold after curing.

FIG. 9 is a perspective partial sectional view of the completed frameportion showing the string holes in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred exemplary tennis racket in accordance with the presentinvention is shown generally at 10 in FIGS. 1-5. The tennis racket frame10 includes a head 12, a handle 14 and neck 16 extending between thehead 12 and handle 14. The head 12 has an arcuate top portion 18 as bestshown in FIG. 2. The head 12 further includes two spaced arcuate sideportions 20 and 22 which extend between the neck 16 and arcuate topportion 18 of head 12.

The neck 16 includes two arcuate members 24 and 26 which extend betweenthe arcuate side portions 20 and 22 and the handle 14. A yoke 28 isprovided extending between the arcuate side members 24 and 26. The yoke28 is preferably a chevron having an apex 30 and two side elements 32and 34 which slope downward and away from the apex 30. Preferably, thecross sectional area of the side elements 32 and 34 increases from theapex 30 towards the point at which the side elements 32 and 34 areconnected to side members 24 and 26, respectively.

As a feature of the present invention, the two side elements 32 and 34should slope downward sufficiently to provide vertical strings of thesame length extending between the yoke 28 and top portion 18. The areaof uniform length vertical strings is highlighted in FIG. 2 by thevertical phantom lines 36 and 38. The two side elements 32 and 34 mayalso be sloped downward sufficiently to provide longitudinal stringswhich increase in length from the apex of the yoke 30 towards the sidemembers 24 and 26.

A chevron shaped yoke 28 is preferred due to its enhanced structuralreinforcement features. Especially preferred is a chevron as shown inFIG. 2 where the cross-sectional area of the chevron increases from theapex 30 towards the neck side members 24 and 26. Other chevron shapesare possible so long as the downward slope of the yoke 28 matches or isgreater than the curvature of the arcuate top portion 18 so that thevertical strings between phantom lines 36 and 38 are of equal length ofgradually increasing length from the apex 30 towards the side members 24and 26. The equal length vertical strings provide uniform elasticresponse to ball impact over the area covered by the equal lengthstrings. This is an improvement over conventional rackets in which thelength of the vertical strings decreases as one moves from the apex 30towards the racket sides 24 and 26.

For both structural and performance consideration, it is preferred thatthe downward sloping elements 32 and 34 slope downward at an angle suchthat the angle between the side elements 32 and 34 and a horizontal line(shown) at 40 in phantom) is between 10 degrees and 45 degrees. Moreparticularly, it is preferred that the downward slope of top surface 33of side elements 32 and 34 relative the horizontal line 40 isapproximately 20 degrees with the downward slope of the bottom surface35 being approximately 25 degrees.

The inverted chevron shape of the yoke 28 provides the additionaladvantage of increasing the space between racket string holes 42, asbest shown in FIG. 5. A problem experienced with prior art rackets isthat the conventional arcuate shape in which the apex of the arc isbelow the attachment points of the yoke to the neck requires that thestring holes be spaced rather close together. The close spacing of thestring holes structurally weakens the yoke and may result in prematureracket failure. As a particular feature of the inverted chevron yokedesign in accordance with the present invention, the string holes in theyoke 28 may be spaced further apart to produce a structurally strongeryoke.

The exemplary tennis racket design shown in FIGS. 1-5 may be made fromany suitable material such as wood, metal, resin-impregnated graphitefibers or other composite material such as resin impregnated boronfilaments, glass fibers, polyamide fibers (Kevlar) or the like. However,as a particular feature of the present invention it is preferred thatthe tennis racket frame be made from polycrystalline metal oxide fibersalone or in conjunction with a core of other fibers such as graphite orfiberglass in a resin matrix. The preferred polycrystalline metal oxidefibers include aluminum oxide, boron oxide and silicon dioxide. Aparticularly preferred fiber composition is a polycrystalline metaloxide fiber which is 52 weight percent aluminum oxide, 14 weight percentboron oxide and 24 weight percent silicon dioxide. This preferredpolycrystalline metal oxide fiber is sold as Nextel™ 312 Ceramic Fibersand is available from 3M Corporation, St. Paul. Minn. It is preferredthat the polycrystalline metal oxide fibers be present in a resin matrixwhich includes resins selected from the group consisting of silicon,epoxy, phenolic and polyamide resins. Epoxy resins are preferred.Additionally, it is preferred that ceramic particles such as boroncarbide, silicon carbide, aluminum oxide, tungsten carbide and titaniumcarbide be added to the polycrystalline metal oxide fiber/resin matrixin order to additionally enhance the abrasion resistance and durabilityof the composite material. The silicon carbide particles areparticularly preferred. Particle sizes ranging form 40 microns to 275microns are suitably incorporated into the ceramic fiber/resin matrix.Processing and curing of the Nextel ceramic fibers is carried outaccording to conventional composite material fabrication techniques withconventional curing temperatures and pressures being utilized.

A nagging problem with both composite material and metal tennis racketsis that plastic grommets must be inserted into the string holes afterfinal fabrication of the racket frame is completed in order to preventstrings from being cut and broken by the racket frame. In accordancewith the present invention, a method is provided for incorporating aprotective material into the racket frame during fabrication to providea protective insert which replaces the grommets and grommet stripscommonly found in both metal and composite material tennis rackets. Thestring protection inserts as described below may be used in conjunctionwith composite rackets made from resin impregnated graphite and othercomposite materials such as resin impregnated boron filaments, glassfibers, Kevlar, and the ceramic fiber/resin matrix described above.

FIG. 6 is a partial perspective/sectional view of a head portion 50 of atennis racket frame prior to curing. The uncured frame section precursor50 includes resin impregnated fibers 52, blowing agent and core material54 and the elastomeric inserts 56. Fabrication of rackets in generalutilizing a composite material shell 52 and central core 54 with blowingagent is well known. A particularly preferred process is disclosed inU.S. Pat. No. 4,413,822, which is assigned to American Sports Equipment,Camarillo, Calif. The contents of this patent are hereby incorporated byreference.

The insert 56 is preferably made from an elastomeric or plastic materialsuch as nylon or similar plastics. The plastic must be stable at thetemperatures which are usually used to cure various composite materials.Further, the plastic should be sufficiently strong and resilient toprovide protection of the racket string from the harder compositematerials while not being so soft that it is easily worn or abraded bythe racket strings. As shown in FIG. 6, it is preferred that the inserts56 extend above and below the outer perimeter of the frame precursor 50.This is to insure that when the frame precursor is placed in the mold,the resin matrix cannot flow over the top or bottom of insert 56. As aparticular requirement, the cross section of the rod shaped insert 56must be larger than the string hole. Preferably, the insert 56 will havea diameter of about 1/4" to 3/8". The string holes can be molded intothe insert during formation of the insert prior to or during molding ofthe frame or the string holes can be drilled through the inserts priorto or after molding of the frame. The string holes are preferably in thecenter of the insert and are around 1/8" in diameter. This will leave amolded in grommet of plastic material between the string and frame aswill be discussed below.

As shown in FIG. 7, the frame precursor 50 along with plastic inserts 56is placed in a suitable mold shown generally at 58. The mold 58 includesleft half 60, right half 62 and center piece 64. As previouslymentioned, the extension of the ends of insert 56 above and below frameprecursor 50 is important so that the resin matrix does not seep betweenthe ends of the mold and the insert 56 during molding. This is importantsince a thin film of resin matrix covering the ends of insert 56 wouldproduce a sharp edge which would cut the racket strings when the racketis strung.

The frame precursor 50 is heated to the desired curing temperature andmaintained there for a sufficient time in accordance with conventionalcomposite material curing techniques to form the cured racket frame 66as shown in FIG. 8. The cured frame 66 includes the cured compositeshell 68, central foam core 70 and a plurality of elastomeric inserts56.

As the final step in preparing the frame structure for stringing, holes72 are preferably drilled through inserts 56. As previously mentioned,the holes 72 may be preformed in the inserts prior to molding of theracket frame, if desired. As can be seen from FIG. 9, the diameter ofinsert 56 is sufficiently large so that the string which is passedthrough string hole 72 is protected from direct contact with thecomposite shell 68 by way of the molded-in insert 56. The provision ofmolding the insert 56 into the cured racket frame 74 provides apermanent and secure string protective mechanism which is an improvementover grommets and grommet strips which are added to the tennis racketafter preparation of the frame. The molded in configuration of theinsert 56 also provides an aesthetically pleasing look and finish to theracket frame not possible with grommets and/or grommet strips.

As another feature of the present racket frame, discrete masses of highdensity material, such as tungsten, lead or dense metal, may bepositioned within the foam core at various locations around the racketframe. The strategic placement of these high density inserts into theracket frame provide for control of racket balance and precise locationof the center of gravity. The inserts are preferably incorporated intothe foam core precursor and molded in place during molding of the racketstructure.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited to the specificembodiments as illustrated herein, but are only limited by the followingclaims.

What is claimed is:
 1. A method for making a .[.tennis.]. racket framehaving a head portion adapted for mounting a plurality of .[.tennis.].strings, said method comprising the steps of:preparing a .[.tennis.].frame precursor from uncured composite material, said frame precursorhaving a head portion; placing string inserts at spaced locations aroundsaid frame precursor head portion, said string inserts being rod shapedand extending perpendicularly through said frame precursor, said insertshaving a cross-sectional area which is larger than the cross-sectionalarea of the .[.tennis.]. strings and being softer than the curedcomposite material; curing said .[.tennis frame.]. precursor with saidinserts located therein for a sufficient time and at a sufficienttemperature to form a composite .[.tennis.]. racket frame having a headportion with said string inserts molded therein; forming holes in saidinserts of sufficient size for said .[.tennis.]. strings to passtherethrough for mounting, said holes being sufficiently small so thatsaid .[.tennis.]. strings are surrounded by said inserts when saidstrings are mounted within the formed holes.
 2. A method according toclaim 1 wherein said rod shaped insert is selected from the groupconsisting of a plastic and an elastomer.
 3. A method according to claim2 wherein said plastic is nylon.
 4. In a method for making a compositematerial .[.tennis.]. racket having a head portion and plastic insertslocated therein with a hole passing through each insert for mounting aplurality of .[.tennis.]. strings, wherein the improvementcomprises:molding said plastic inserts into said head portion duringcuring of said composite material racket frame to provide a curedcomposite material racket frame having a plurality of plastic insertstherein; and forming holes through said molded in .[.elastomerc.]..Iadd.elastomeric .Iaddend.inserts to provide string mounting holeshaving a molded in plastic protective grommet between the string andsaid composite material frame through which said string is mounted tosaid head portion.
 5. A .[.tennis.]. racket frame comprising a headportion, a handle portion and a neck portion extending between said headand handle portions and wherein said head portion is adapted to bestrung; said frame comprising a shell comprising polycrystalline metaloxide fibers in a resin matrix wherein each of said metal .[.and.].oxide fibers consists essentially of aluminum oxide, boron oxide andsilicon dioxide.
 6. A .[.tennis.]. racket according to claim 5 whereinsaid polycrystalline metal oxide fibers are 62 weight percent aluminumoxide, 14 weight percent boron oxide and 24 weight percent silicondioxide.
 7. A .[.tennis.]. racket according to claim 5 wherein saidresin matrix is selected from the group consisting of .[.silicon.]..Iadd.silicone.Iaddend., epoxy, phenolic and polyamide resins.
 8. A.[.tennis.]. racket according to claim 7 wherein said resin matrix isepoxy.
 9. A .[.tennis.]. racket according to claim 5 wherein said resinmatrix includes ceramic particles selected from the group of particlesconsisting of boron carbide, silicon carbide, aluminum oxide, tungstencarbide and titanium carbide.
 10. A .[.tennis.]. racket according toclaim 9 wherein said ceramic particles have particle sizes of between 40microns and 275 microns.
 11. A .[.tennis.]. racket frame according toclaim 5 wherein said frame further includes a foam core which issurrounded by said shell.
 12. A .[.tennis.]. racket frame according toclaim 5 wherein said shell further includes graphite fibers or glassfibers in said resin matrix.